Control and prevention of caving and sliding in drilling oil and gas wells



Fatente d May E6, 1944 CONTROL AND PREVENTION OF SAVING AND SLHDlNG IN DRIHJLING OIL AND GAS WELLS 3 Claims.

The present invention relates to the drilling of oil and gas wells, where detrimental caving or sliding of shale and sand are likely to be encountered.

This application is-a continuation in part of a copending application Ser. 351,188-filed Aug. 3,

The objects of the invention are to control and prevent such caving and sliding.

It has been found that muds in common use, such as the bentonites, actually encourage such sliding and caving, because they form hydrated films surrounding the formation particles. Such highly hydrated films act as lubricants, thus decreasing the angle of repose of the formation material and allowing more shale and sand to slide into the hole. Even the supposed strength of gelled bentonite is insufficient to hold these ma terials in place.

The present invention is based on the discovery that cohesive and adhesive properties can be measured by the pressure required to force the filter cake through a small orifice, together with an estimation of its ductility and that when such a pressure exceeds lbs., with a 3 inch orifice, the cohesive-adhesive and ductile character of the mud is sufllcient to control or preventsliding or caving.

The addition to drilling mud. of compounds of the pyrophyllite type which are not excessively hydrated, provides the cohesive-adhesive-ductile character necessary to prevent caving and sliding.

These materials are characterized by an internal structure whose residual binding forces are such as to prevent the formation of thixotropic gel. On the other hand, such clay as native clay, kaolin, fullers earth, or thermally decomposed clays lack sufflcient hydration to give them the necessary adhesive-cohesive-ductile character.

The invention takes cognizance of the fact that there are three known types of hydration; first the wetting of the surface, second the hydration of the adsorbed ions, third the growth of a filamentous mass which encloses much water and produces a thixotropic solution.

Only hydration of the first and second kind together will produce the necessary adhesion-cohesion-ductility characteristics.

The accompanying diagram illustrates the structure of an alumino-silicate capable of acquiring hydration only of the first and second kind.

By the addition of such a material to drilling mud, it is possible to make it sufilciently adhesive, cohesive, and ductile to prevent caving and sliding of shale and sand formations.

For example, in a well 30 miles northwest of Oklahoma City, where caving or sliding was occurring and hampering operations, and where the mud consisted of a bentonite native clay mixture with a filter cake extrusion pressure of 5 lbs., 1000 lbs. of pyrophyllitic clay, with the illustrated structure was added to each .bbls. of mud; This brought the filter cake extrusion pressure up to 15 lbs., and the caving and sliding stopped. On removing this 15 lbs. extrusion pressure mud and returning to the use of bentonite the condition of caving and sliding recurred. Readdition of like amounts of this pyrophyllitic clay again stopped the objectionable caving and sliding.

The pyrophyllite used in the example given was obtained from deposite in Rush County, Kansas. The illustration given was derived from X-ray diffraction data obtained from this pyrophyllite and shown in this table:

that disclosed in copending patent application Ser. No. 351,188 filed Aug. 3, 1940, in that itinvolves a pyrophyllitic clay of cohesive, adhesive, ductile characteristics for controlling and preventing caving and sliding in drill holes, whereas the invention of the other patent application re- 

